Apparatus and methods for concentrating platelet-rich plasma

ABSTRACT

Apparatus and methods for concentrating platelet-rich plasma is described herein. One variation may generally comprise a tube having a length and defining a channel within and one or more ports located at a proximal end of the tube and in fluid communication with the channel. A plunger may slidably translatable within the channel while forming a seal against an inner surface of the channel and a float may have a pre-selected density and defining a concave interface surface, wherein the float is slidably contained within the channel such that the concave interface surface is in apposition to the one or more ports.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/773,583 filed Jan. 27, 2020, which claims the benefit of priority toU.S. Prov. Apps. 62/802,031 filed Feb. 6, 2019 and 62/851,803 filed May23, 2019, each of which is incorporated herein by reference in itsentirety and for any purpose.

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for separatingblood components. More particularly, the present invention relates toapparatus and methods for effectively separating and removing specificcomponents from blood.

BACKGROUND OF THE INVENTION

Blood may be fractionated and the different fractions of the blood usedfor different medical needs. For instance, anemia (low erythrocytelevels) may be treated with infusions of erythrocytes. Thrombocytopenia(low thrombocyte (platelet) levels) may be treated with infusions ofplatelet concentrate.

The sedimentation of the various blood cells and plasma is based on thedifferent specific gravity of the cells and the viscosity of the medium.When sedimented to equilibrium, the component with the highest specificgravity (density) eventually sediments to the bottom, and the lightestrises to the top. Under the influence of gravity or centrifugal force,blood spontaneously sediments into three layers. At equilibrium the top,low-density layer is a straw-colored clear fluid called plasma. Plasmais a water solution of salts, metabolites, peptides, and many proteinsranging from small (insulin) to very large (complement components).Plasma per se has limited use in medicine but may be furtherfractionated to yield proteins used, for instance, to treat hemophilia(factor VIII) or as a hemostatic agent (fibrinogen). The term plateletrich plasma (PRP) is used for this component because most of the plasmaproteins and platelets in the whole blood are in the plasma followingslow centrifugation so the concentration of platelets in the plasma hasincreased while suspended in supernatant plasma. The uppermost layerafter centrifugation typically contains plasma proteins only and istypically called platelet-poor plasma (PPP) due to the absence or lownumber of platelets as a result of a “hard spin”.

The bottom, high-density layer is a deep red viscous fluid comprising anuclear red blood cells (RBC) specialized for oxygen transport. The redcolor is imparted by a high concentration of chelated iron or heme thatis responsible for the erythrocytes high specific gravity. Packederythrocytes, matched for blood type, are useful for treatment of anemiacaused by, e.g., bleeding. The relative volume of whole blood thatconsists of erythrocytes is called the hematocrit, and in normal humanbeings can range from about 38% to about 54%.

The intermediate layer is the smallest layer, appearing as a thin whiteband on top the erythrocyte layer and below the plasma, and is calledthe buffy coat. The buffy coat itself has two major components,nucleated leukocytes (white blood cells) and anuclear smaller bodiescalled platelets (or thrombocytes). Leukocytes confer immunity andcontribute to debris scavenging. Platelets seal ruptures in the bloodvessels to stop bleeding and deliver growth and wound healing factors tothe wound site. The buffy coat may be separated from whole blood whenthe blood is subjected to a “hard spin” in which the whole blood is spunhard enough and long enough so that platelets sediment from plasma ontopacked red cells and white cells percolate up through red cell pack tothe interface between red cells and plasma.

When whole blood is centrifuged at a low speed (e.g., up to 1,000 g) fora short time (e.g., two to four minutes) white cells sediment fasterthan red cells and both sediment much faster than platelets. At higherspeeds the same distribution is obtained in a shorter time. The methodof harvesting PRP from whole blood is based on this principle.Centrifugal sedimentation that takes the fractionation only as far asseparation into packed erythrocytes and PRP is called a “soft spin”which is typically used to describe centrifugation conditions underwhich erythrocytes are sedimented but platelets remain in suspension.“Hard spin” is typically used to describe centrifugation conditionsunder which erythrocytes sediment and platelets sediment in a layerimmediately above the layer of erythrocytes.

The auto-transfusion equipment used to make autologous plateletconcentrates requires a skilled operator and considerable time andexpense and these devices require a large prime volume of blood. Whilemany of these devices have somewhat reduced the cost and the timerequired, skilled operators and time are still required. Accordingly,there remains a need for simple and effective methods and devices forseparating and removing components from whole blood.

SUMMARY OF THE INVENTION

The present invention relates to apparatus and methods for rapidfractionation of blood into its different components, e.g., erythrocyte,plasma, and platelet fractions. The design described herein for a buffycoat concentrator should provide platelet and white blood cell (WBC)yields comparable to other gravitational platelet separation (GPS)designs. The manufacturing costs should be lower and are easy to use. Italso allows for the user to choose a desired level of buffy coatconcentration. Markings on the tube can be provided to indicate theamount of platelet-depleted plasma (PPP) to be withdrawn prior toresuspension of the buffy coat to yield a desired concentration factor(the more PPP removed, the higher the float will be within the tube andhence the higher the concentration). Because platelets sediment onto athin layer of red blood cells (RBC) trapped within the float concavity,platelet damage should be minimal and resuspension should be easier (aswith GPS) than when platelets are sedimented directly onto a hardsurface.

One variation may include a tube having a single port or severalseparate ports at a proximal end of the tube. A sterile vent may bedefined at a distal end of the tube and a plunger may be slidablypositioned within the tube. A tuned density float having a concavitydefined in the upper float surface may be slidably positioned within thetube between the plunger and the ports. An inner surface of the tubedistal to the ports and facing the interior of the tube may define aconvex ceiling which mirrors the concavity of the upper float surface ina corresponding manner.

During use with one variation, the float and plunger may be positionedat a top of the tube and ready for introduction of anticoagulated blood.The float and plunger may slide down distally within the tube in unisonas blood is introduced through the blood introduction port. Displacedair within the tube may escape through the sterile vent at the bottom oftube. The float may rise to an equilibrium position with the bulk of RBCpacking out beneath the float and plasma PPP above the float. Hence thefloat may be configured to have its density tuned to automaticallyposition itself between the two layers under centrifugation. A buffycoat overlaying a small volume of RBC may be trapped within the convexupper surface of the float.

After centrifugation, a volume of PPP may be withdrawn through the PPPport and the float and plunger may slide up the tube in unison. Thevolume of PPP remaining above float will determine the level of buffycoat concentration in the final buffy coat concentrate. Air may enterthe space beneath the plunger via the sterile vent at the bottom of tubeto replace volume of PPP removed.

A syringe may be attached to the buffy coat withdrawal port and a smallvolume of air may be introduced via the syringe to facilitateresuspension of the buffy coat. The tube may be inverted multiple timesor shaken or swirled to resuspend the buffy coat in the remaining PPP.After resuspension, the buffy coat concentrate may be withdrawn into thesyringe, ready for use.

In order to provide for a consistent, non-selectable, predeterminedconcentration factor, the float or a protrusion therefrom may beconfigured to collide with the ceiling of the tube or to encounter somesort of mechanical stop shy of the tube ceiling in order that duringproduct withdrawal, the float comes to rest at a position which resultsin a fixed volume of fluid being retained above the float.

Under this scenario, the volume above the float cannot readily beremoved after PPP withdrawal unless a sterile vent is provided in theupper section of the tube to allow air to replace the volume of fluidharvested. The upper space (between the plunger and tube ceiling) cancontain anticoagulant so that whole blood may be drawn directly from thepatient into the device and be admixed with anticoagulant as it fillsinto the upper space.

The detachable plunger pull rod can optionally be used to create avacuum within the tube by pulling and locking it in a fully extendedposition with the vent blocked and the blood introduction port comprisedof a septum or equipped with a closed closable valve. Anticoagulant canoptionally be preloaded into the space above the plunger in order thatit be admixed with freshly drawn whole blood as it is introduced.

For certain applications, a less concentrated PRP completely devoid ofRBC and WBC may be desirable. For such applications, a flat-topped floatmay be preferable. To produce a PRP without other contaminating celltypes, the device may be spun under conditions (“soft spin”) whichsediment the more massive RBC and WBC beneath the buoy while plateletsremain suspended above. Aside from removal of unwanted RBC and WBC, theplatelet fraction would be concentrated only to the extent that thevolume of RBC were removed (e.g., if the hematocrit were 50%, removingthe RBC would potentially increase platelet concentration in the plasmaby about two-fold over that in whole blood).

In another variation, the plunger and float may be positioned in aninitial state and a vent may be capped or plugged and the upper spaceoptionally containing anticoagulant. The blood (or anticoagulated blood)may be introduced either by pulling the plunger or collecting directlyfrom the patient after evacuation of the tube (e.g., by pulling plungerto full extension and locking) through withdrawal port. The detachableplunger may then be detached and the withdrawal port capped.

After centrifugation, the buffy coat may form above the lowermost levelof float at its equilibrium position. The PPP may be withdrawn (withvent stoppered) where the float and plunger travel in unison. The PPPwithdrawal may stop when a portion of the float collides with tubeceiling. With the vent unstoppered, a syringe may be attached to thewithdrawal port and a volume of air may be introduced (to facilitateresuspension). The buffy coat may be resuspended in the retained volumeof PPP. The resuspended buffy coat may be withdrawn into a collectionsyringe, and the extracted volume replaced by air via the sterile vent.

In yet another variation, a protrusion, such as a peg, may project fromthe bottom of the float and a corresponding opening may be defined alonga top surface of the plunger to snugly receive the protrusion into theopening. This allows for the float to couple to the plunger temporarilyso that when the plunger in pulled down to introduce anticoagulatedblood or anticoagulant followed by whole blood directly from thepatient, the float and plunger may move down in unison. The fit of theprotrusion into the plunger opening may be tight enough that they remaincoupled when the plunger is pulled down. The buoyant force on the floatduring centrifugation is sufficient to urge the protrusion out of theopening so that the float 24 can rise and eventually find itsequilibrium position in the separated blood.

One variation of a separation apparatus described herein may generallycomprise a tube having a length and defining a channel within and one ormore ports located at a proximal end of the tube and in fluidcommunication with the channel. A plunger may slidably translatablewithin the channel while forming a seal against an inner surface of thechannel and a float may have a pre-selected density and defining aconcave interface surface, wherein the float is slidably containedwithin the channel such that the concave interface surface is inapposition to the one or more ports.

One variation for separating components from blood may generallycomprise displacing a float and a plunger from a first position within atube to a second position by introducing a volume of blood into the tubeand applying a centrifugal force to the volume of blood contained withinthe tube such that the blood forms at least a first fractional layer anda second fractional layer and the float positions itself between thefirst and second fractional layers, wherein the float has a pre-selecteddensity and defines a concave interface surface. Once centrifuged, atleast the first fractional layer may be withdrawn from the tube via atleast one port defined at a proximal end of the tube such that the floatand plunger are moved from the second position back towards the firstposition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of one variation of a separationassembly having a float with a concavity defined on an upper surface.

FIGS. 2A to 2E show perspective views of one variation for use of thetube.

FIGS. 3A to 3F show side views of another variation for use of the tube.

FIGS. 4A to 4D show side views of various float configurations.

FIG. 5 shows a perspective view of yet another variation of the tubewhere the plunger and float may incorporate a temporary securementmechanism.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the description, terms such as “top”, “above, “bottom”,“below” are used to provide context with respect to the relativepositioning of components when, e.g., a container tube with fractionalcomponents of blood are positioned when the longitudinal axis of acontainer tube is positioned upright or non-horizontally. Suchdescription is used for illustrative purposes only.

As discussed herein, when sedimented to equilibrium, the component withthe highest specific gravity (density) eventually sediments to thebottom, and the lightest rises to the top. Under the influence ofgravity or centrifugal force, blood spontaneously sediments into threelayers. At equilibrium the top, low-density layer is a straw-coloredclear fluid called plasma. The term platelet-rich plasma (PRP) is usedfor this component because most of the plasma proteins and platelets inthe whole blood are in the plasma following slow centrifugation so theconcentration of platelets in the plasma has increased while suspendedin supernatant plasma. The bottom, high-density layer comprisessedimented red blood cells (RBC). The intermediate layer, if the bloodis subjected to further centrifugation, is called the buffy coat.

The present invention relates to apparatus and methods for rapidfractionation of blood into its different components, e.g., erythrocyte,plasma, and platelet fractions. The design described herein for a buffycoat concentrator should provide platelet and white blood cell (WBC)yields comparable to other gravitational platelet separation (GPS)designs. The manufacturing costs should be lower and are easy to use. Italso allows for the user to choose a desired level of buffy coatconcentration. Markings on the tube can be provided to indicate theamount of platelet-depleted plasma (PPP) to be withdrawn prior toresuspension of the buffy coat to yield a desired concentration factor(the more PPP removed, the higher the float will be within the tube andhence the higher the concentration). Because platelets sediment onto athin layer of red blood cells (RBC) trapped within the float concavity,platelet damage should be minimal and resuspension should be easier (aswith GPS) than when platelets are sedimented directly onto a hardsurface.

The perspective view of FIG. 1 and the illustrated step-by-stepdescription herein are sufficient to permit appreciation of theprinciple and method of operation. Some variants described may also beuseful for certain applications/markets and can allow also forpreparation of WBC-free and RBC-free platelet-rich plasma (PRP). Onevariation 10 is shown as having a tube 12 which may have three separateports 16 at a proximal end of the tube 12, as shown, e.g., bloodintroduction port 18, PPP withdrawal port 22, and a buffy coatwithdrawal port 20. A sterile vent 30 may be defined at a distal end ofthe tube. The plunger 14 may be slidably positioned within the tube andhave a bottom circumferential seal 32 form a seal against the inner wallof the tube as well as optionally form a seal against the tube floorduring centrifugation when in its lowermost position within the tube 12.This optional seal 32 may provide an extra level of assurance inpreventing any blood from leaking out of the bottom of the tube underpressure developed during centrifugation.

A tuned density float 24 having a feature, for example, a flattenedsurface, a convex surface, or a concavity 26 defined in the upper floatsurface may be slidably positioned within the tube between the plunger32 and the ports 16. The lower float surface 34 may also be configuredto have various shapes, e.g., a flattened surface, a concave surface, ora convex surface. Alternatively, the lower float surface 34 may betapered to present a sloped conical shape or a sloped surface angledfrom one side of the float 24 towards the opposite side of the float 24.An inner surface 28 of the tube distal to the ports and facing theinterior of the tube may define a convex ceiling which mirrors theconcavity 26 of the upper float surface in a corresponding manner.

While three separate ports 16 are indicated for introduction of citratedwhole blood, removal of PPP and harvest of buffy coat concentrate, asingle port could be sufficient. Generally, multiple use of a singleport is generally frowned upon, so more than the three ports shown mayalso be incorporated and used, if desired.

FIGS. 2A to 2E show perspective views of one variation for use of thetube as described. FIG. 2A shows an initial condition of the tube 12where the float 24 and plunger 14 are positioned at a top of tube andready for introduction of anticoagulated blood. FIG. 2B shows the float24 and plunger 14 slide down distally within the tube in unison as bloodBL is introduced through the blood introduction port 18. Displaced airwithin the tube 12 may escape through the sterile vent 30 at the bottomof tube. FIG. 2C illustrates how under centrifugation, the float 24rises to an equilibrium position with the bulk of RBC packing outbeneath the float and plasma PPP above the float. Hence the float 24 maybe configured to have its density tuned to automatically position itselfbetween the two layers under centrifugation. For example, the float 24may have a density which is tuned specifically for use with whole blood,e.g., 1000 to 1100 kg/m³ (or specific density of 1.0 to 1.1 at 25° C.),while in other variations, the float 24 may be fabricated to have adifferent density, e.g., 1.03 to 1.07, etc. A buffy coat overlaying asmall volume of RBC is trapped within the convex upper surface of thefloat.

FIG. 2D shows the tube after centrifugation where a volume of PPP iswithdrawn through the PPP port 22. As the PPP is withdrawn, the float 24and plunger 14 may slide up the tube 12 in unison. The volume of PPPremaining above float 24 will determine the level of buffy coatconcentration in the final buffy coat concentrate. Air may enter thespace beneath the plunger 14 via the sterile vent 30 at the bottom oftube 12 to replace volume of PPP removed.

A syringe may be attached to the buffy coat withdrawal port 20 and asmall volume of air may be introduced via the syringe to facilitateresuspension of the buffy coat BC, as shown in FIG. 2E. The tube 12 maybe inverted multiple times or shaken or swirled to resuspend the buffycoat BC in the remaining PPP. After resuspension, the buffy coat BCconcentrate may be withdrawn into the syringe, ready for use.

In order to provide for a consistent, non-selectable, predeterminedconcentration factor, the float or a protrusion therefrom may beconfigured to collide with the ceiling of the tube or to encounter somesort of mechanical stop shy of the tube ceiling in order that duringproduct withdrawal, the float comes to rest at a position which resultsin a fixed volume of fluid being retained above the float.

Under this scenario, the volume above the float 40 cannot readily beremoved after PPP withdrawal unless a sterile vent 46 is provided in theupper section of the tube 12 to allow air to replace the volume of fluidharvested. The vent 46 is depicted in FIGS. 3A to 3F as a side portwhich can be blocked by attachment of a cap or plug 50. The figures alsoillustrate a detachable plunger pull rod 52 which can be used to drawblood into the device via the (center) blood introduction port 44 (incontrast to forcibly injecting blood from a syringe into theintroduction port). The upper space (between the plunger 14 and tubeceiling) can contain anticoagulant so that whole blood may be drawndirectly from the patient into the device and be admixed withanticoagulant as it fills into the upper space.

The depicted detachable plunger pull rod 52 can optionally be used tocreate a vacuum within the tube 12 by pulling and locking it in a fullyextended position with the vent blocked and the blood introduction portcomprised of a septum or equipped with a closed closable valve.Anticoagulant can optionally be preloaded into the space above theplunger in order that it be admixed with freshly drawn whole blood as itis introduced.

For certain applications, a less concentrated PRP completely devoid ofRBC and WBC may be desirable. For such applications, a flat-topped floatmay be preferable. To produce a PRP without other contaminating celltypes, the device may be spun under conditions (“soft spin”) whichsediment the more massive RBC and WBC beneath the buoy while plateletsremain suspended above. Aside from removal of unwanted RBC and WBC, theplatelet fraction would be concentrated only to the extent that thevolume of RBC were removed (e.g., if the hematocrit were 50%, removingthe RBC would potentially increase platelet concentration in the plasmaby about two-fold over that in whole blood).

FIGS. 3A to 3F show the variation in which the plunger 14 and float 40are positioned in an initial state, as shown in FIG. 3A. The vent 46 isshown as capped or plugged 50 and the upper space optionally containinganticoagulant. FIG. 3B illustrates how blood BL (or anticoagulatedblood) may be introduced either by pulling the plunger 14 or collectingdirectly from the patient after evacuation of the tube (e.g., by pullingplunger to full extension and locking) through withdrawal port 44. Thedetachable plunger may then be detached and the withdrawal port 44capped 48.

After centrifugation, as shown in FIG. 3C, the buffy coat BC may formabove the lowermost level of float 40 at its equilibrium position. ThePPP may be withdrawn (with vent stoppered), as shown in FIG. 3D, wherethe float 40 and plunger 14 travel in unison. The PPP withdrawal maystop when a portion of the interface surface 42 of the float 40 collideswith tube ceiling. With the vent 46 unstoppered, as shown in FIG. 3E, asyringe may be attached to the withdrawal port 44 and a volume of airmay be introduced (to facilitate resuspension). The buffy coat BC may beresuspended in the retained volume of PPP. The resuspended buffy coat BCmay be withdrawn into a collection syringe, as shown in FIG. 3F, and theextracted volume replaced by air via the sterile vent.

The float 40 shown in this embodiment may be configured to becylindrically shaped and having an angled interface surface along itsupper surface which contacts the PPP and BC after centrifugation.Although shown with the present tube embodiment, the float 40 may beused with any of the variations described herein. While the upperinterface surface 42 of the float 40 may define a relatively flattenedsurface, the interface surface 42 may instead define any number ofconfigurations. In any of these variations where the upper float surfaceis concave, flattened, or convex, a layer may be applied to the upperfloat surface which is relatively slippery. In one variation, a siliconelayer may be formed upon the upper float surface to facilitate theremoval of platelets from the upper surface.

As shown in the side view of FIG. 4A, the upper float interface surface42 may be angled relative to a longitudinal axis 60 of the float 40 suchthat the surface preferentially slides platelets along a predefineddirection. The angle Θ defining the slope of the interface surface 42 asthe angle defined between a transverse plane of the float 40 and theinterface surface 42 may range anywhere from 0 degrees to just under 90degrees, but may range in some embodiments from, e.g., 2 to 60 degrees.Having the angled interface surface 42 may also allow for the bloodcomponents such as the BC to preferentially pool or collect between thetube interior wall and along the interface surface 42 allowing for easeof collection when the float is positioned near or at the upper interiorsurface of the tube, as shown in FIG. 3F above.

FIG. 4B shows a side view of another variation of the float 62 which maydefine an angled interface surface 64 which terminates at a flattenedshould 66. FIG. 4C shows a side view of another variation of float 68where the interface surface 70 may be defined as a concave surface whichis asymmetrically angled to slope preferentially towards one side of thefloat 68, as shown. FIG. 4D shows a side view of yet another variationof float 72 where the interface surface 72 may be defined as a convexsurface which is also asymmetrically angled to slope preferentiallytowards one side of the float 72, as shown.

Another embodiment is illustrated in the perspective view of FIG. 5 inwhich a protrusion 80, such as a peg, may project from the bottom of thefloat 24. A corresponding opening 82 may be defined along a top surfaceof the plunger 14 to snugly receive the protrusion 80 into the opening82. This allows for the float 24 to couple to the plunger 14 so thatwhen the plunger 14 in pulled down to introduce anticoagulated blood oranticoagulant followed by whole blood directly from the patient, thefloat 24 and plunger 14 may move down in unison. Any of the floatvariations described herein may be utilized with this embodiment or anyof the other embodiments as described.

If desired, anticoagulant can first be pulled into the tube 12 and then(with all ports 16, including the vent port, capped) the plunger 14 canbe pulled all the way down to create a vacuum and locked in place (e.g.,by a twist engaging a latch between pull rod 52 or plunger 14 and tube12). Then a blood line stuck into the patient's vein can be attached tothe tube 12 either via a septum port or Luer port with valve (so thatthe vacuum is maintained within the tube 12 until the blood line isconnected).

Alternatively, an anticoagulant may be preloaded in the tube 12 so thatthe user could merely pull a vacuum using the pull rod 52 and thenconnect a blood line to the patient (without needing to addanticoagulant).

The fit of the protrusion 60 into the plunger opening 62 may be tightenough that they remain coupled when the plunger 52 is pulled down. Thebuoyant force on the float 24 during centrifugation is sufficient tourge the protrusion 60 out of the opening 62 so that the float 24 canrise and eventually find its equilibrium position in the separatedblood.

The apparatus and methods disclosed above are not limited to theindividual embodiments which are shown or described but may includecombinations which incorporate individual features between the differentvariations. Modification of the above-described assemblies and methodsfor carrying out the invention, combinations between differentvariations as practicable, and variations of aspects of the inventionthat are obvious to those of skill in the art are intended to be withinthe scope of the claims.

What is claimed is:
 1. A separation apparatus, comprising a tube havinga length and defining a channel within; one or more ports located at aproximal end of the tube and in fluid communication with the channel;and a float having a pre-selected density and an interface surfacedefined by a first interface portion and a second interface portionextending from the first interface portion, wherein the float isslidably contained within the channel such that the interface surface isin apposition to the one or more ports, and wherein the first interfaceportion defines a first angle relative to a transverse plane of thefloat and the second interface portion defines a second angle relativeto the transverse plane such that the second angle is greater than thefirst angle whereby the interface surface is configured topreferentially pool blood components along the second interface portion.2. The apparatus of claim 1 wherein the first or second angle relativeto the transverse plane is between 2 to 60 degrees.
 3. The apparatus ofclaim 1 wherein the one or more ports comprise a blood introductionport, a PPP withdrawal port, or a buffy coat withdrawal port.
 4. Theapparatus of claim 1 further comprising a plunger slidably positionedwithin the tube.
 5. The apparatus of claim 4 further comprising a pullrod attachable to the plunger.
 6. The apparatus of claim 1 wherein thefloat has a density between a layer of RBC and a layer of PPP when thetube is centrifuged.
 7. The apparatus of claim 1 wherein the float has aspecific density between 1.0 to 1.07.
 8. The apparatus of claim 1wherein the interface surface comprises a concave interface surfacewhich is configured to retain a buffy coat layer when the tube iscentrifuged.
 9. The apparatus of claim 1 wherein the float comprises aportion extending from the float for contact against an upper innersurface of the tube.
 10. The apparatus of claim 1 wherein the interfacesurface is angled such that the blood components are preferentiallypooled between the inner surface of the channel and the interfacesurface.